The present invention relates to ground-penetrating radars and coal mining, and more particularly to methods and systems for radio-imaging anomalous geology in coal bed deposits.
Given the growing need to produce cleaner run-of-mine (ROM) coal, improved information about the seam geology and coal quality in coal mine operations is of great value. The identification of anomalies is important so planning operations keep productivity high and cut cleaner coal. For example, the identification of a paleochannel anomaly before mining began would allow longwall panels to be laid out to avoid crossing it.
A natural coal seam waveguide occurs in layered sedimentary geology because the electrical conductivity of the bounding shale, mudstone, and fire clay, ranges between 0.01 and 0.1 Siemens per meter (S/m) (100 and 10 ohm-meters). Inside, the conductivity of the coal is near 0.0005 S/m (2,000 ohm-meters). The 10:1 conductivity contrast enables the waveguide travel of electromagnetic waves within the coal bed.
The electric field (Ez) component of a traveling electromagnetic wave (EM) is polarized in a vertical direction and the magnetic field (Hy) component is polarized horizontally in the seam. The energy in this part of the EM wave travels laterally in the coal seam from the transmitter to the radio imaging receiver. There is a horizontally polarized electric field (Ex) that has zero value in the center of the seam and reaches maximum value at the sedimentary rock-coal interface. This component is responsible for transmission of the electromagnetic wave signal into the boundary rock layer. The energy in this part of the EM wave travels vertically in the coal deposit.
The magnitude of coal seam radiowave decreases as it travels along the waveguide. The attenuation rate and cylindrical spreading of wave energy in the coal seam are two of the things at work that attenuate the travelling signals. The cylindrical spreading factor is       1          r        ,
where r is the distance from the transmitting to receiving antenna. This factor compares with the non-waveguide far-field spherically spreading factor of       1    r    .
Thus, for a given separation of one-hundred meters, the magnitude of the seam EM wave decreases by ten in a waveguide, and by a factor of one-hundred in an unbounded media. So one advantage of sending signals down a seam waveguide is the much greater travel distance. Another advantage is that the traveling electromagnetic wave predominantly stays within the coal seam, the main item of interest.
A coal-seam electromagnetic wave is very sensitive to changes in the waveguide geometry and materials. The radiowave attenuation rate (decibels per 100 feet) and phase shift (electrical degrees per 100 feet) were determined by Dr. David Hill at the National Institute for Science and Technology (NIST). Dr. James Wait was the first to recognize that natural waveguides exist in the earth""s crust. Both are Fellows in the Institute of Electrical and Electronic Engineers. The science underlying the traveling of an electromagnetic wave in the coal seam waveguide is well known. The engineering of both the crosshole transmitter and receiver has also been developed to a high degree of performance. The transmitter and receiver are synchronized to enable the measurement of total path phase shift from the transmitter to the receiver location. The total phase shift measurement is a distinguishing factor in the radio imaging-IV instrumentation. Prior art radio imaging instrumentation measures only the change in magnitude of radiowave, e.g., attenuation, when propagating in the coal seam waveguide.
The path length or distance a radio signal travels can be determined from attenuation measurements. In uniform-construction waveguides, the path is a straight line. The straight line path is an assumption used in the Algebraic Reconstruction Technique (ART) tomography algorithm. But radiowaves are refracted near significant geologic anomalies causing the travel path of the radiowave to bend and be longer than in the uniform waveguide case. This bending cannot be accounted for in ART processing and accounts for this distortion in the ART tomography processing algorithm. By measuring the total path phase shift, the bending effect can be accounted for in a new type of tomography reconstruction algorithm called Full-wave Inversion Code (FWIC) radio imaging IV acquires data that can be processed in the Sandia National Laboratories"" WAIC algorithm. The effect of attenuation in the waveguide is to reduce the magnitude of the electromagnetic wave along the path.
Under sandstone sedimentary rock, the attenuation rate increases because more of the radio imaging signal travels vertically into the boundary rock, i.e., leaks from the waveguide. If water is injected into the coal, then clay in the coal causes the electrical conductivity to decrease and the attenuation rate/phase shift to increase.
The attenuation rate/phase shift rapidly increases with decreasing seam height. This coal seam thinning can be easily detected with radio imaging. The above graphical presentation of coal seam waveguide attenuation and phase constants represents the science factor in the art and science of interpreting radio imaging tomographic images. Higher attenuation rate zones suggest that either the coal seam boundary rock is changing, the seam is rapidly thinning, or/and water has been injected into the coal seam. Drilling and radar would determine the exact cause of the anomalous seam condition. This advance in the state of the art in mining would reduce both risk and cost in coal extraction.
Faults and dykes cause reflections to occur in the waveguide. The reflections can appear as excess path loss. Total phase shift measurements are useful in detecting reflection anomalies.
The predominating electromagnetic wave propagation mode in layers of coal is a xe2x80x9cseam wavexe2x80x9d. Such is polarized in the plane of the seam, and has a uniform, polarized electric field orthogonal to the layer. In horizontal lying coal bed layers, the magnetic field will be horizontally polarized with the same field strength across a vertical cross-section. The electric field is vertically polarized. A third electric field is polarized in the horizontal plane and is maximum value at each boundary of the seam.
The horizontal component of the electric field is null near the physical center of the coal seam, albeit if the lower-resistivity boundary layers above and below are about equal in their respective material electrical resistivities.
The present inventor, Larry G. Stolarzyck, has described methods and equipment for imaging coal formations in geologic structures in many United States Patents. Some of those Patents are listed in Table I, and are incorporated herein by reference.
In underground coal mining practice, horizontal magnetic dipole antennas can be driven by a radio transmitter so a seam wave will propagate within the coal, or other layer of higher-resistivity media. A remote, horizontal magnetic dipole receiving antenna is then used to measure the seam wave with a receiver synchronized to the transmitter. Fiber-optic cables are preferably used for the receiver-transmitter synchronization, e.g., because a metallic cables would interfere with reception by receiving the transmitted signals, and re-radiating them to compete with the direct signal to the receiver. Phase coherent receiver design allows synchronous detection and accurate phase measurements of the direct signal. The effects on direct signal phase help elicit the nature of the coal layer, given a priori or concomitant material dielectric-constant measurements.
However, the logistics of providing the synchronization cable can prove impossible in some mines and in some applications. So it would be desirable to synchronize such transmitters and receivers without requiring a cable between the receiver and transmitter.
It is therefore an object of the present invention to provide an instrument for the detection and high-resolution imaging of anomalous geologic structures.
It is another object of the present invention to provide an imaging method for simultaneously transmitting a synchronizing and an imaging electromagnetic wave from a single location for phase coherent detection by a receiver.
It is still another objective of the present invention to provide a method of receiver phase coherency with a transmitter and thereby obtain maximum receiver threshold sensitivity when measuring total attenuation and phase shift of imaging electromagnetic waves passing through a geologic target.
It is still another object of the present invention to provide a full-wave inversion code method in the reconstruction of imaging of anomalous geology when the ray path assumption in the algebraic reconstruction algorithm becomes invalid.
Briefly, a coal bed anomaly detection and imaging embodiment of the present invention comprises a synchronous transmitter and receiver that are separated by a geologic structure with embedded and hidden anomalies. The transmitter sends out two signals from magnetic dipole antennas. Such signals are widely separated in frequency but synchronized internally in the transmitter to one another. The higher frequency is used to make phase shift and attenuation measurements at the receiver by synchronous detection. The lower frequency is used at the receiver to synchronize the receiver to the transmitter. The higher frequency signal is measurably affected by anomalies in the intervening geologic structure. The lower frequency signal is fixed low enough so it is not substantially affected by the intervening geologic structure. Geologic modeling tools are preferably downloaded by geoscientists to their personal computers. The total attenuation and phase shift measurements are plugged into a full-wave inversion code (FWIC) process. A hypothetical model is uploaded for processing by a forward solver so the nature of the anomalous geologic structure can be estimated. A resulting reconstructed image of the anomalies in silhouette is then downloaded for interpretation of the image by the geoscientist.
An advantage of the present invention is a system is provided that eliminates the need for a synchronization cable wired between a transmitter and a receiver.
Another advantage of the present invention is a method is provided for the measurement of the total attenuation and phase shift of an imaging electromagnetic wave after its having propagated through a geologic target.
A further advantage of the present invention is an image reconstruction code is provided in a method that is not invalidated by refraction, reflection, and scattering of an imaging electromagnetic wave in a geologic target.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.